A variety of filtering apparatus has been employed for the filtration of liquids such as coolants utilized in machining operations. One widely used type of system is the traveling belt filter, which lends itself to automation and permits a wide variety of filter media to be utilized. The media may be in the form of an expendable roll of cellulosic and/or polymeric material or it may be of a permanent type in the form of an endless belt which is cleaned at the point of dirt discharge by air blow-off or washer headers. These permanent media preferably consist of a square mesh polymeric mono-filament material.
The most common type of system utilized in coolant filtration is the intermittent traveling belt type, which may employ either a permanent or disposable media. A disposable media is generally carried on a relatively coarse endless perforate belt which does not perform a filtering function. The build-up of sludge on this type of filter forms a filter cake 1/8th inch to 2 inches high, and the filter cake itself acts as a fine filter which captures the dirt as the liquid penetrates the depth of the filter cake. However, the accumulation of the filter cake results in an increase of resistance to liquid flow through the media and the resulting pressure differential between the clean and dirty sides of the filter media can be one-half to twenty pounds per square inch. When a predetermined pressure drop has been sensed by the control system, liquid flow through the filter is interrupted and the media is indexed. During this indexing and cleaning of the filter media, the flow of filtered liquid is maintained from a large clean liquid well or simply by by-passing dirty coolant.
When the filter construction is in the form of a filter box submerged in a tank of liquid to be filtered and wherein the liquid is caused to flow through the filter media into an opening in the filter box by means of the differential pressure across the filter media, a significant factor contributing to the efficiency of filtration is the effectiveness of the seal between the filter media and the surface of the filter box. Particularly as the filter cake accumulates on the filter media, with resulting increased resistance to flow, any leakage path by which dirty liquid can by-pass the filter by entering the filter box from around the edges of the filter media, rather than through the filter media, will substantially reduce filtration efficiency.
This sealing problem is magnified when the filter is intended for machining coolant filtration applications, where the sludge which normally builds up on the filter media must be conveyed out of the unit. Because the filter box is submerged in the dirty liquid, it is necessary to have the filter media climb a ramp extending upwardly from the filter box to a higher elevation above the liquid level, where the accumulated sludge can be discharged outside the tank. These ramps generally are at about a 45 degree angle, and therefore angle flights connected to and spanning the gap between a pair of parallel chains are commonly used to help convey this sludge up the ramp.
Fabric type filter media are preferred in this type of filtration system, because stainless steel mesh belts are subject to fatigue and breakage as a result of the repeated flexing and bending of the screen as it passes around driving drums. Furthermore, stainless steel belts are extremely costly, and require elaborate slack take-up provisions resulting from differential expansion or elongation between the stainless steel belt and the driving chains.
While fabric filter belts are less expensive and easier to install, they produce their own problems. The repeated alternating exposure to liquid and air causes fabrics to shrink, a problem generally compensated for by installing an initially over sized belt. The system must also be able to counteract wrinkles that occur in the fabric. Furthermore, the low specific weight of the fabric requires means for forcibly holding the fabric down against the periphery of the filter box, to prevent by-passing and leakage, as discussed above. The flights which extend across the width of the filter box provide some hold-down effect, but their spacing of approximately one foot does not effectively hold down the lateral longitudinal edges of the fabric belt. It is difficult to have a resilient seal traveling with the edges of the belt because such seal must traverse the same drive sprockets or drums which the filter belt loop traverses. Stationary seals relying upon engagement with the intermittently moving belt are subject to wear and possible tearing of the fabric belt.
In my prior U.S. Pat. No. 3,087,620, a disposable filter media was both held down against the vacuum box and frictionally advanced by means of a pair of laterally spaced chains. Neither the filter media nor the chains circumscribed the filter box, and the chains did not positively connect to or drive the filter media or any supporting conveyor. In fact, a media or conveyor-driving chain could not effectively be used to also hold down the filter media because either (1) the necessity of a positive driving connection between the chain and supporting conveyor would preclude the overlying relationship between the chain and media carried on the conveyor, or (2) the media could not pass between the chain and its driving sprockets without becoming enmeshed in the sprocket and probably torn. The impracticality of such a hold-down chain would be particularly evident if used in an installation where the filter media and driving chain circumscribe a submerged filter box.
Accordingly, it is the principal object of the present invention to provide an improved sealing system for preventing leakage around the filter belt in a system of the type employing an endless filter belt surrounding a submerged filter box.